Magnetic recording is the preferred method of information storage when data must be capable of being read immediately after writing or where the data is to be processed by a machine. The recording medium usually consists of fine, magnetizable particles which are dispersed in a polymeric binder and coated onto a support. In order to work efficiently, magnetic media must possess certain physical and magnetic properties, e.g., the medium surface must be sufficiently smooth to enable accurate reading of the recorded signal by the magnetic head. It must also be free from asperities and other surface roughness that can lower the signal to noise ratio. In addition, the medium must be durable, so that it is possible to record and reproduce information repeatedly. To be durable, the magnetic particles must be firmly bound to the support and not be worn off by the passing of the magnetic head over the media. This durability must persist under all environmental conditions under which the medium is to be used. In addition, it is important that the media have sufficient lubricity so that the magnetic head passes freely over the surface with the minimum coefficient of friction. Furthermore, the medium should have uniform physical properties over a wide temperature range.
Magnetic recording media are normally produced by passing a non-magnetic support through an apparatus which coats the support with a liquid dispersion of the magnetic medium. This dispersion consists of a binder, in either an uncured or solvated state, having the magnetizable particles homogeneously dispersed therein. After coating, the dispersion dries or cures to give a tough binder film having the magnetizable particles uniformly distributed throughout. The desired physical and magnetic properties of the recording layer are often dictated by certain processing parameters. One important parameter is good pigment wetting in the bulk liquid dispersion. Good pigment wetting is closely related to dispersion stability and is essential to ensure an even distribution of pigment throughout the finished magnetic coating.
Another important processing parameter is the so-called `pot-life` of the dispersion, the time for which the dispersion can be kept at a sufficiently low-viscosity before coating. Most conventional, magnetic media rely on a chemical reaction between a polyfunctional isocyanate crosslinking agent and hydroxy functionality on the binder material to cure the coating and so toughen it. The isocyanate is generally added to the dispersion prior to coating (known as an activation step) and consequently the dispersion has a finite pot-life. if the cure reaction is too fast, the resultant short pot life creates time constraints on the coating process and can make it difficult to obtain sufficiently smooth coatings. If the cure reaction is too slow, the magnetic coating will have poor green strength until the cure reaction has progressed sufficiently. As a result, the magnetic coating will be susceptible to damage during subsequent processing steps unless an inconvenient and expensive time delay is built into the manufacturing process. Furthermore, the need for an activation step (or post-coating cure step as in e-beam crosslinked systems) complicates and thus increases the cost of media production.
The bulk of the binder materials used in the preparation of conventional media are of relatively low molecular weight. Consequently, a cure reaction is essential to produce media having the appropriate mechanical properties. A further problem inherent in this approach arises from the very presence of these low molecular weight species. If the cure reaction, for any reason, is incomplete, then low molecular weight species can remain in the coated media following curing. Such species can plasticize the media leading to poor media durability. The low molecular weight species may prematurely gel or cause flocculation of the magnetic particles resulting in non-uniform and unacceptable magnetic performance of the coatings. Furthermore, low molecular weight species left after curing may migrate to the surface of the media and come into contact with the recording head where they can adversely affect performance through increased friction, stiction, head clogging and/or poor blocking resistance.
Another problem encountered with conventional binder systems is the degradation of the binder material through hydrolysis or oxidation. This leads to an increase in the amount of low molecular weight species in the binder matrix and hence to an increased occurrence of the problems described above.
Many known polymeric binder systems exhibit a change in physical properties over a range of temperatures. This change is caused by the glass-transition temperature (Tg) of the chosen binder material(s) falling within the temperature range in which the media is to be used. Magnetic media are often exposed to a wide range of temperatures, often over a range in excess of 100.degree. C. In an extreme case a binder system can go from a stiff, brittle material at a low temperature, to a soft, tacky material as the temperature is increased. Such changes in physical properties can cause performance problems during use of the media.
In view of the above deficiencies in prior art magnetic media binders, there is a need to provide a binder suitable for use in magnetic media which is comprised of high molecular weight starting materials and which does not require polymerization or crosslinking to form a suitable magnetic coating. In addition, it is desirable that these high molecular weight binder materials provide good pigment wetting and dispersion stability, including systems with high pigment loadings. It is also desirable that the magnetic media have good mechanical properties, such as smoothness, durability and lubricity, along with good electrical properties, such as signal-to-noise ratio. It is also desirable that the physical and magnetic properties of the media are relatively uniform throughout a wide range of temperatures.
International Patent Publication No. W090-14662 discloses an improved magnetic recording medium comprising a nonmagnetic support with at least one magnetic layer comprising a magnetizable pigment dispersed in a binder composition comprising a block copolymer of a hard polymer having a glass-transition temperature of at least 70.degree. C. and a soft polymer having a glass-transition temperature of less than -30.degree. C.
The manufacture of such block copolymers in which one or more blocks of a hard, non-elastomeric polymer is/are bonded to one or more blocks of a soft, elastomeric polymer is known. Depending on the content of the hard and soft polymer blocks in the copolymer, the block copolymer will exhibit non-elastomeric or elastomeric properties. Successive polymerization of the monomers results in block copolymers having a linear structure. If such linear block copolymers are coupled to one another by polyfunctional reactive compounds, branched block copolymers having a star-shaped structure result. The term "star" describes the structure of a multi-arm polymer with copolymer arms which are joined together at a nucleus formed of a coupling moiety or linking agent which is effectively a point relative to the overall size of the remainder of the polymer structure. Methods of preparing such "star" block copolymers are disclosed, for example, in U.S. Pat. Nos. 3,985,830 (Fetters et al), 4,086,298 (Fahrbach et al), 4,180,530 (Bi et al) and 4,780,367 (Lau et al).
The block copolymers disclosed in International Patent Publication No. W090-14662 are of the following general formula: EQU A-B-Y---B-A).sub.z ;
wherein:
Y is a single bond or a multifunctional coupling agent;
A is a hard polymeric segment having a glass-transition temperature of greater than about -70.degree. C.;
B is a soft polymeric segment having a glass-transition temperature of less than about -30.degree. C., and
z is about 1 to 15.
Although the block copolymers of the above general formula may have from 1 to 15 arms, the only binders exemplified comprise either:
(i) a mixture of a linear styrene-butadienestyrene block copolymer (z=1), commercially available from Shell under the trade name KRATON 1101 and a styrene-butadiene star block copolymer (z=3,4 and 6), commercially available from the Phillips Chemical Co., under the trade name KR-01, or
(ii) the linear block copolymer used alone.
KR-01 consists of styrene-butadiene star block copolymers comprising 65% by weight styrene and 35% by weight butadiene. Each individual star block copolymer comprises at least 3 arms represented by (A-B) linked to and projecting from the central moiety represented by Y. However, in practice, preparations of star block copolymers comprise a heterogeneous population of molecules. That is, the number of arms for the population as a whole is a number average value. KR-01 consists of star block copolymers having predominantly 3 arms with a small quantity of 4 and 6 arm star structures. The number average value for the number of arms has been determined to be close to 3.